Add PlayerExit() Hook

I decided to make a new hook in SRB2 called PlayerExit() which requires no arguments. By the way, if you want to use functions that affect players make sure you use for player in players.iterate(). I made the following test Lua script for this new feature: playerexit_test.lua. By the way, I tested this Lua script, and this new feature worked like it is supposed to.

I'm not sure if you were aware, but a similar PlayerQuit hook was made by Prisima about a year ago in !127 (merged). It was not merged because we are not adding new features to 2.1 patches. It may be considered for 2.2 though.

For this reason I'm closing this merge request too for now.

(On another note, wow, I just realised we can't see what Prisima did for his/her own PlayerQuit hook, given the branch was deleted afterwards apparently)

closed

added Lua label